The present invention relates generally to electrical circuits and, more particularly, to a preamplifier and an associated method for amplifying an input signal.
Operational amplifiers (op amps) are utilized in various types of analog circuits. Op amps are typically utilized to provide high voltage gain, high input impedance, and low output impedance. A traditional two-stage op amp includes an input stage coupled to receive a differential input and an output stage that provides an output for the op amp according to the gain of the respective stages.
For example, one common use for op amps is in analog-to-digital converters (ADCs), which convert an analog input into a corresponding digital output signal that can include one or more bits of data. An ADC is typically implemented in an integrated circuit or chip, although it can be implemented on a circuit board by an appropriate arrangement of components. ADCs further can be utilized in a variety of applications, such as instrumentation applications, level detection applications, drivers for LCD screens, servo tracking, and both wired and wireless communications applications.
By way of further example, a pipelined data converter is among the fastest ADC architectures. Among the ADCs adopted in (intermediate frequency) IF-sampling receivers for wireless communication systems, the pipelined converter provides a fair compromise between power consumption and speed. The supply of the converter is being progressively scaled down to 3.3V and beyond, as mandated for by the overall IC miniaturization and perspective usage in System-on-a-Chip (SoC) designs, which exacerbates the thermal budget for the circuit.
To implement the high gain and speed usually required by the design of the residue stages, pipeline ADCs often employ multi-stage op amp structures, such as two stage op amps. Typically, a low-gain pre-amplifier is adopted as a first stage. The low-gain first stage provides several advantages. One popular choice of first stage is the differential couple with diode load. This scheme does not require a common-mode network to be fed back from the second stage, which, if implemented, has the possibility of adversely affecting the common-mode stability. Additionally, the structure can be digitally controlled to regulate the gain of the overall structure.
When designing a two-stage op amp, for example, it is desirable for the first stage to provide a stable output to the second stage. A stable output from the first stage enables the second stage to be designed for near optimum conditions. However, in circumstances where the output of the first stage is unstable, such as in response to temperature variations, the second stage typically needs to be designed to accommodate such instability. This tends to result in reduced performance in the overall op amp structure.
The following presents a simplified summary of the invention in order to provide a basic understanding of some aspects of the invention. This summary is not an extensive overview of the invention. It is intended to neither identify key or critical elements of the invention nor delineate the scope of the invention. Its sole purpose is to present some concepts of the invention in a simplified form as a prelude to the more detailed description that is presented later.
The present invention relates generally to a preamplifier that is substantially resilient to temperature and input-common mode variations. The preamplifier, for example, can be implemented as a first stage of a multi-stage op amp, and which provides an intermediate output to a next stage of the op amp. The preamplifier system includes a feedback network coupled to regulate a common mode voltage of the preamplifier.
For example, the feedback network can be connected between a tail node and a low voltage of the preamplifier, such as to control the current provided (e.g., sourced or sunk) relative to the tail node, based on the common mode voltage. According to one aspect of the present invention, the feedback network can include even a single transistor (e.g., an N-type metal oxide semiconductor field effect transistor NMOSFET). The feedback can be facilitated by employing downshifting circuitry coupled to increase the output impedance of the preamplifier.